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  Java Practices -> Use enums to restrict arguments
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<div class='page-title'>Use enums to restrict arguments</div>

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<br><a href="TopicAction2161-2.html">Type-safe enumerations</a> are useful for constraining
arguments which must take a small set of values, such as a <tt>boolean</tt>
or a restricted set of <tt>String</tt>s or <tt>int</tt>s.
<p>As an example from the servlet API, recall this method for the creation
of an <tt>HttpSession</tt> :
<p><tt>aRequest.getSession(true);</tt>
<p>Any reader who is not familiar with the specification of this method
will be <i>forced</i> to look up the meaning of the <tt>boolean</tt> literal
before they can understand the method call. An alternative style is to
define a well named variable which explains the intent :
<p><tt>boolean CREATE_IF_ABSENT = true;</tt>
<br><tt>aRequest.getSession(CREATE_IF_ABSENT);</tt>
<p>However, the caller is not <i>compelled</i> to use such a style. If
a type-safe enumeration is used for the argument, then the caller is indeed
forced into a style which has greater clarity.
<p><tt>aRequest.getSession(HttpServletRequest.CreateIfAbsent.YES);</tt>
<p>Here, <tt>CreateIfAbsent</tt> would be a static nested type-safe enumeration
with two elements, <tt>YES</tt> and <tt>NO</tt>. This style has these advantages :
<ul>
<li>
the method is forced to be more intelligible at the point of call</li>

<li>
the need to look up the specification is greatly reduced</li>

<li>
it serves as a reminder to those who are already familiar with the specification</li>

<li>
it has compile-time safety (not an issue with a <tt>boolean</tt> argument,
but important if the argument is a <tt>String</tt> or <tt>int</tt>)</li>
</ul>

This style can be a bit verbose. It might be one of those rare occasions where
a <a href='TopicActionda46-2.html'>static import</a> of the enum class is appropriate, 
to avoid using fully qualified names.

<P><b>Example</b>
<P>Here is a stub class illustrating an alternate form of <tt>createSession</tt> :
<br>
<PRE>

<span class='keyword'>import</span> javax.servlet.http.HttpSession;

<span class='comment'>/** Clarify method call with enum argument.  */</span>
<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> HttpRequestWithEnum {

  <span class='comment'>/**
  * Example of calling the method with an enum arg.
  */</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>void</span> main(String... aArgs){
     HttpRequestWithEnum request = <span class='keyword'>new</span> HttpRequestWithEnum();
     HttpSession session = request.createSession(CreateIfAbsent.YES);
  }
  
  <span class='keyword'>public</span> HttpSession createSession(CreateIfAbsent aCreateIfNeeded){
    <span class='keyword'>return</span> <span class='keyword'>null</span>; <span class='comment'>//stubbed out
</span>  }
  
  <span class='comment'>/** The enumeration itself.  */</span>
  <span class='keyword'>public</span> <span class='keyword'>enum</span> CreateIfAbsent {YES, NO};
}
 
</PRE>
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<div class='topic-section'>See Also :</div>
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  <a href='TopicAction2161-2.html?Id=1'>Type-Safe Enumerations</a> <br>
 
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